1. Technical Field of the Invention
The present invention relates generally to a helicopter rotor blade and more particularly to a forward sweep, low noise rotor blade.
2. Discussion of the Related Art
Blade-vortex interaction (BVI) noise is one of the most objectionable sources of helicopter noise. This impulsive blade-slap noise can be particularly intense during low-speed landing approach and maneuvers. Over the years, a number of flight and model rotor tests have examined blade tip modification and other blade design changes to reduce this noise.
Impulsive rotor noise is generally caused by localized aerodynamic events on the blade. At high advancing tip speeds, it is due to the presence of transonic flow and associated shocks about the blade tip region. For this, blade tip speed and thickness effects are the most important parameters in defining the noise mechanism and thus its control. At lower speeds, where BVI noise dominates, the number of important parameters increases because the complexity of the rotor wake and its embedded tip vortices becomes central to the problem.
The important interactions on the advancing side of the rotor occur in the first quadrant (between azimuth angles of .psi.=0 to 90 deg.) where blades interact with tip vortices which were shed upstream. The resulting BVI noise is most strongly directed upstream on the advancing side below the rotor. Strong BVI can also occur on the retreating side in the fourth quadrant, with noise directed below and downstream of the rotor. The local parameters which govern the intensity and directivity of BVI noise include the strength and core size of the tip vortex at the time of interaction, the interaction geometry (angles and "miss distances") of the blade and vortex and the interaction Mach numbers. Of course, in turn, these parameters are determined by operational requirements, such as rotor thrust and flight conditions, and design parameters, such as rotor speed, number of blades, and blade section (i.e. chord width), plan form, twist and tip shape.
One early BVI noise reduction approach taken was to modify the blade tip vortex structure by tip shape design changes. While this approach was unsuccessful, general lessons learned through theoretical and experimental work were used to improve the blade designs. The newer design rotors were generally four-bladed with lower tip speed, thinner blade sections and swept tips. The effect was to decrease thickness and high speed impulsive noises and, to a lesser extent, the BVI noise. Much of the progress in recent years has been in areas other than blade design. For example, flight operation procedures, such as changes in landing speed and descent rate for fixed glide slope approach, were found to reduce main rotor impulsive noise levels some 2 to 5 dB. Rotor tip speed reduction was used successfully on a light helicopter to reduce noise 3 to 4 dB by reducing rotor speed to 90 percent of standard rpm. Another method employed higher harmonic control (HHC) of blade pitch to reduce BVI noise. Results have shown up to 6 dB BVI noise reduction depending on operating condition and control phase. Although the progress of these studies is impressive, the operational and active control methods may be difficult to apply in practice. Also, for many rotorcraft, they may be less effective than passive blade and tip design changes.
It is accordingly an object of the present invention to provide a rotor blade which reduces rotorcraft noise.
It is a further object of the present invention to provide a rotor blade which reduces blade-vortex interaction (BVI) noise.
It is a further object of the present invention to provide a rotor blade which reduces BVI noise in a passive manner.
It is a further object of the present invention to provide a rotor blade which is applicable to standard helicopters and tiltrotors.
It is yet another object of the present invention to accomplish the foregoing objects in a simple manner.
Additional objects and advantages of the present invention are apparent from the drawing and specification that follow.